Austenitic nickel/chrome/cobalt/molybdenum/tungsten alloy and use thereof

ABSTRACT

The invention relates to an austenitic nickel/chrome/cobalt/molybdenum/tungsten alloy, comprising (in mass %): carbon: 0.05 to 0.10%; chromium: 21 to 23%; cobalt: 10 to 15%; molybdenum: 10 to 11%; aluminium: 1.0 to 1.5%; tungsten: 5.1 to 8.0%; yttrium: 0.01 to 0.10%; boron: 0.001 to 0.010%; titanium: max. 0.50%; silicon: max. 0.50%; iron: max. 2%; manganese: max. 0.5%; nickel: remainder; including unavoidable impurities caused by the smelting process.

[0001] The invention relates to an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy.

[0002] It is known that nickel master alloys are widely used in thermal engines, such as for example internal combustion engines, stationary and flying gas turbines, because they have extremely good mechanical properties, in particular in the temperature range comprised between 700 and 900° C.

[0003] Many of these alloys contain substantial portions of γ′ and γ″ phases, which are decisive for increasing the strength in the temperature range comprised between 700 and 900° C. For the precipitation of the γ′ and γ″ phases, the alloy elements aluminium, titanium, tantalum and niobium are mostly used.

[0004] Thus, GB-A 929,687 describes a nickel-chrome-cobalt-molybdenum alloy, which significantly increases its strength by γ′ and γ″ precipitation by means of adding 2.0-3.5% titanium, 0-0.8% aluminium and 2-5.25% niobium.

[0005] Herein, up to 10 atomic % of tungsten are allowed as substitution element of molybdenum and up to 3% by mass of tantalum are proposed as substitution element of niobium.

[0006] GB-A 1,036,179 essentially describes a limitation of the elements mentioned in GB-A 929,687 only, with the exception that iron is no more allowed as alloying element.

[0007] U.S. Pat. No. 4,981,644 also describes a nickel-chrome-cobalt-molybdenum alloy, which is precipitation-hardening by means of titanium, aluminium, niobium and tantalum, but which only prescribes a total content of 1.0-8.5% of the elements rhenium, molybdenum and tungsten.

[0008] WO-A 90/03450 describes a precipitation-hardening alloy, for which, in contrast to U.S. Pat. No. 4,981,644, not only upper limits, but also lower limits regarding the precipitation-hardening elements Al, Ti, Nb and Ta are defined; in contrast thereto, only upper limits of rhenium, hafnium and vanadium are mentioned.

[0009] The wish to be able to use highly heat resisting alloys also as wrought alloys, i.e. for example also as sheet metals, which can be deformed and joined, led to the solid solution hardened alloys, which do not contain more than about 2% by mass of precipitation-hardening elements, such as for example aluminium, titanium and niobium.

[0010] Thus, GB-A 1,336,409 describes a nickel-chrome-cobalt-molybdenum alloy, in which the sum of the precipitation-hardening elements aluminium and titanium is limited to 0.8-2.1%.

[0011] U.S. Pat. No. 4,877,461 describes a nickel-chrome-molybdenum-cobalt alloy having an improved creep strength, which approximately contains the same high level, namely 0.5-2.25%, but also an addition of up to 5% tungsten.

[0012] EP-B 0633 325 discloses an alloy, which, based upon U.S. Pat. No. 4,877,461, prescribes a limitation of 0.5-2.0% by mass for aluminium and titanium and maximum 5% by mass for the addition of tungsten, but also comprises 0.7-2.5% by mass tantalum, which is advantageous for the strength.

[0013] It may have been achieved to produce heat resisting, deformable and joinable sheet metal materials by reducing the aluminium and titanium content to 0.5-2.0% by mass and adding tungsten and tantalum to nickel-chrome-cobalt-molybdenum alloys, but the obtained heat resistance was always clearly lower than the one of the precipitation-hardening alloy.

[0014] It is thus the object of the present invention to improve nickel-chrome-cobalt-molybdenum alloys of the initially mentioned type, such that with sufficient oxidation stability, the values of the heat resistance and creep strength are improved in such a way that the service life of items made of such alloys will be significantly increased. The material shall be usable as wrought alloy, which can be deformed and welded.

[0015] This aim is achieved by an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy consisting of (in % by mass): carbon: 0.05 to 0.10% chrome: 21 to 23% cobalt: 10 to 15% molybdenum: 10 to 11% aluminium: 1.0 to 1.5% tungsten: 5.1 to 8.0% yttrium: 0.01 to 0.10% boron: 0.001 to 0.010% titanium: max. 0.50% silicium: max. 0.50% iron: max. 2% manganese: max. 0.5% nickel: remainder

[0016] including unavoidable impurities caused by the smelting process.

[0017] A preferred austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy has the following composition (in % by mass): carbon: 0.05 to 0.10% chrome: 21 to 23% cobalt: 12 to 13% molybdenum: 10 to 11% aluminium: 1.0 to 1.5% tungsten: 5.1 to 7.0% yttrium: 0.04 to 0.07% boron: 0.001 to 0.005% titanium: max. 0.50% silicium: max. 0.50% iron: max. 2% manganese: max. 0.5% nickel: remainder

[0018] including unavoidable impurities caused by the smelting process.

[0019] In contrast to the state of the art, the nickel-chrome-cobalt-molybdenum-tungsten alloy according to the invention comprises clearly more than 5% by mass tungsten and does not have any addition of tantalum for improving the heat resistance and creep strength.

[0020] Furthermore, the addition of 0.01 to 0.10% by mass yttrium improves the cyclic oxidation stability.

[0021] Alloys having additions of only up to 5% by mass tungsten, even with low additions of tantalum, do not reach the strength potential, which can be obtained by solid solution hardening. Surprisingly it has been found, in contrast to the teaching of U.S. Pat. No. 4,877,461 and EP-B 0633 325, that tungsten contents of more than 5% represent a very efficient possibility to increase the strength. But for a given composition, the addition of tungsten is limited to 8.0% by mass, since with higher contents the hot formability is no longer given.

[0022] Due to its excellent creep strength in the temperature range comprised between 700 and 900° C. the nickel-chrome-cobalt-molybdenum-tungsten alloy according to the invention is especially suitable for items, such as for example

[0023] Stationary and flying gas turbines

[0024] Combustion engines

[0025] Components in steam turbines

[0026] Components of progressive gas and steam power stations

[0027] Turbo-superchargers

[0028] High temperature resistant ventilators

[0029] The mentioned items can be easily formed of the material according to the invention, since it is not only highly suitable for hot forming, but also for cold forming operations—such as for example cold rolling to thinner dimensions, folding, deep-drawing, edging.

[0030] Due to the good weldability of the material, the joining of larger components is also possible without any problems.

[0031]FIGS. 1 through 4 clearly show the advantage of the alloys 1-4 according to the invention compared to the alloys 5-7 representing the state of the art, by means of the service lives obtained at 700° C., 750° C., 800° C. and 850° C. in the stress rupture test.

[0032] The composition of the alloys 1-7 is represented in table 1.

[0033] In order to not only compensate the negative effect of tungsten on the cyclic oxidation stability, but also to further improve the cyclic oxidation stability, yttrium has been added by alloying to the alloys according to the invention. The content according to the invention of 0.01 to 0.10% results from the minimum content of 0.01% which is absolutely necessary for a visible improvement effect and from the defined maximum content of 0.10%, beyond which an “overdoping” and thus a decrease of the cyclic oxidation stability will probably take place.

[0034] The positive effect of yttrium is confirmed by the clearly lower material increase represented in FIGS. 5-6, which show the cyclic oxidation test at 700° C. and 800° C. during a test period of more than 1000 hours. TABLE 1 elements in % alloys according to the invention alloys according to the state of the art by mass alloy 1 alloy 2 alloy 3 alloy 4 alloy 5 alloy 6 alloy 7 C 0.065 0.074 0.057 0.067 0.079 0.067 0.090 Cr 21.90 21.75 21.75 22.33 21.95 22.15 22.05 Co 12.50 12.35 12.45 12.61 11.65 11.95 11.65 Mo 10.3 10.1 10.15 9.79 8.88 9.54 8.85 Ti 0.43 0.43 0.42 0.43 0.43 0.40 0.43 Al 1.28 1.16 1.18 1.08 1.18 1.18 1.16 Nb — — — — — — — Fe 0.91 0.92 0.99 0.05 0.10 1.15 0.13 B 0.003 0.002 0.002 0.004 0.001 0.002 0.004 Zr — — — — — — — Ni remainder remainder remainder remainder remainder remainder remainder W 5.6 5.1 5.2 7.9 — 3.03 0.04 Ta — — — — — — — Y 0.01 0.01 0.07 0.04 — — — 

1. Austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy consisting of (in % by mass): carbon: 0.05 to 0.10% chrome: 21 to 23% cobalt: 10 to 15% molybdenum: 10 to 11% aluminium: 1.0 to 1.5% tungsten: 5.1 to 8.0% yttrium: 0.01 to 0.10% boron: 0.001 to 0.010% titanium: max. 0.50% silicium: max. 0.50% iron: max. 2% manganese: max. 0.5% nickel: remainder

including unavoidable impurities caused by the smelting process.
 2. Austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy consisting of (in % by mass): carbon: 0.05 to 0.10% chrome: 21 to 23% cobalt: 12 to 13% molybdenum: 10 to 11% aluminium: 1.0 to 1.5% tungsten: 5.1 to 7.0% yttrium: 0.04 to 0.07% boron: 0.001 to 0.005% titanium: max. 0.50% silicium: max. 0.50% iron: max. 2% manganese: max. 0.5% nickel: remainder

including unavoidable impurities caused by the smelting process.
 3. Use of an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy according to claim 1 or 2, in particular for combustion chambers of stationary or flying gas turbines.
 4. Use of an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy according to claim 1 or 2, in particular for compressors and turbo-superchargers of combustion engines.
 5. Use of an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy according to claim 1 or 2, in particular for steam turbines.
 6. Use of an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy according to claim 1 or 2, in particular for shafts, roots and blades of high temperature ventilators.
 7. Use of an austenitic nickel-chrome-cobalt-molybdenum-tungsten alloy according to claim 1 or 2 for gas and steam turbine power stations, in particular combustion chamber walls, blades, tubes, boiler walls, membrane walls, collectors or the like. 